The present invention relates to a method for simulating a workpiece contour by simulating a milling operation as well as a corresponding device, in particular a simulation computer.
With CNC-controlled processing machines, such as milling machines, a workpiece is typically encoded either directly or the workpiece is modeled first with a CAD system and then converted into an equivalent CNC parts program. The resulting CNC parts program and/or the CAD model then correspond to the idealized processing commands for the processing machine. The CNC program is loaded into a CNC controller, and the processing machine is controlled according to the CNC program.
If the workpiece produced according to the CNC program is within the desired manufacturing tolerances of an ideal workpiece, then this method poses no problems. However, if the manufactured workpiece does not satisfy the requirements, then the process should be optimized to determine which changes are necessary, for example in the CNC program, to produce a perfect workpiece.
It may be possible to change sequentially individual processing commands and/or individual operating parameters of the processing machine, and to produce a new workpiece and then to test the newly produced workpiece. This approach, however, is very cumbersome and also expensive as well as material- and time-consuming. This is particularly the case because the causes for the deviations between the actually produced workpiece and the intended workpiece are often not known.
For this reason, mechatronic systems, such as processing machines, are more frequently simulated. This is particularly true for milling machines and the workpieces produced with milling machines. In order to analyze the result of such simulation, a visualization environment is required and hence a process for realistically simulating the surface of a workpiece obtained by simulation.
In particular with simulation of milling processes, the visualization component is quite important. This helps to represent data and results, such as milling points, milling paths, and coordinate systems, as well as the workpiece contours calculated by the simulation system.
Today's visualization components are capable of three-dimensional rendering and can be used interactively. The displayed content can be orthographic and/or perspective 3-D projections which can be changed interactively the user. The user is thereby able to rotate, translate and zoom the displayed content. By selecting certain partial content, easy to use visualization components can also produce associated information, such as dimensions, spatial arrangement or correspondence with other partial content of the scene.
Together, this leads to a better understanding of the production process. Moreover, the surface quality of the workpiece to be machined can be readily determined and analyzed in the design stage, allowing optimization of the current parameter set that controls and drives the machine tool.
This makes it possible to produce and machine a “virtual workpiece”, so that an actual workpiece does not have to be produced. Moreover, the processing machine doesn't even have to exist.
To make quantitative statements concerning the expected workpiece contour, a high-quality visualization is hence essential. The virtual workpiece contour should preferably be rendered as a 3-D model. A representation of the milling points and paths using just points and lines is therefore not sufficient. The workpiece contour should be computed across an area from the milling data. This requires a method for producing a surface grid, which is supplied to the visualization component and subsequently rendered.
Known methods for visualizing a workpiece surface typically process only interpolated milling points, i.e., the travel of the tool center.
In the simplest case, a surface grid can be obtained by linearly interpolating the milling points. For example, conventional grid algorithms compute a triangular grid. The algorithm includes the interpolation as well as other characteristic properties, such as the accuracy of the milling path and the contour. Disadvantageously, this method uses only the milling points which typically describe the tool center points (TCP) to calculate the surface grid. With a cylindrical cutter, the TCP—which is sometimes also referred to as tool tip—represent those points where the longitudinal axis of the tool penetrates the workpiece surface. The tool center point is calculated as the point displaced by the tool radius correction (WRK). For spherical cutters, the TCP and the center of the sphere are typically identical.
This models a point-shaped cutter (cutter with a dimension zero). It will be understood, that this model does reflect a realistic milling processes. For example, if the cutter has a diameter of 12 mm, then the location of the actual milling cut is displaced by 6 mm from the position of the cutter center point.
Disadvantageously, the geometry of a cutter is not taken into account in the simulation. To obtain realistic workpiece contours, the geometry of the cutter should therefore be considered. Conventional methods that take the cutter geometry into account correspond more closely to the original, but cannot compete due to delay caused by their internal data structures and the long computing times.
The known algorithms for solving this problem are therefore either imprecise in their approximation of the contour or inadequate in their performance.
It would therefore be desirable to provide a method based on the initial milling data, which enables a sufficiently precise simulation of the expected workpiece contour with acceptable computing times. In addition, the cutter geometry should be taken into account without unduly impairing the computing time.